Kinetic Studies on the Oxidation of Substituted Phenols in Wastewater by Hydrogen Peroxide Activated by Polyoxometalates Catalysts.

Abstract

The rate of oxidation of substituted phenols in wastewater was studied by reaction of phenols with polyoxometalate catalysts to activate H2O2. The substituted phenols used in this study are, phenol, para-Cresol, meta-Cresol, para-Chlorophenol, and para-nitrophenol. The polyoxometalates used are, H3PW12O40 (HWPA), ph-H3PW12O40 (ph-HWPA), H3PMo12O40 (HMoPA), and Na5Mo10V2PO40(HMoVPA). The catalyzed rate and catalyzed rate constants were calculated for these catalytic reactions. meta-Cresol has the highest value of rate constant, while para-nitrophenol has the lowest value. The rate of these reactions increases with increasing the concentration of H2O2, catalyst, and substituted phenol. Also, increase in temperature increases the rate of these reactions while increase in ionic strength increases them only slightly. Hammet equation was used to study the effect of the presence of electron donating and withdrawing groups on phenol on the rate of these reaction. Electron donating groups (eg. -CH3 group) on phenol increase the rate of these reactions, but electron withdrawing groups (NO2, and CL groups) on phenol decrease the rate. Also, the reaction constant (ρ) was calculated for these reactions by using Hammet equation and it was observed to be negative value, which indicates that, the reaction follows free-radical mechanism. Eyring equation was used to calculate the enthalpy of activation for these reactions. HWPA has the lowest value of enthalpy of activation (∆H# =38.7 kJ/mole), but HMoVPA has the highest value ((∆H# =110.8 kJ/mole). It means that, HWPA is the most reactive catalyst, while HMoVPA is the lowest reactive among the catalysts studied. q The effect of oxygen (air) was studied. It was found that it has no effect on the rate of these reactions. The rate of these reactions decreases with increase in acidity, and also with increase in volume percentage of Acetonitrile. Immidazole decreases the rate of these reactions. Water was found to be the best solvent for these reactions. HWPA was the most effective catalyst among the group of catalysts studied, for complete degradation of substituted phenols in wastewater. GC-Mass spectroscopy and IR - spectroscopy were used to identify the products of these reactions, which were CO2, H2O, and some organic acids. Methacrylic acid test was used to prove that, the reaction follows free-radical mechanism.